For me, the crux is simply here: "What income do you have (gross/net)? Both tax class I with €2,400 / €2,000 net. For me, as a step backward after changing industry as an entry-level salary with potential upwards, for my partner, rather small increases are expected within experience levels or collective agreement adjustments. For my girlfriend, it will come down to about €1,400 if she works part-time after parental leave." You cannot build a house in that price category with this. No advice like "better build now before it gets more expensive" helps.
Exactly that is the point. Such a change can only fail if I am suddenly privately overwhelmed with building. What if, for €2,000 (the step back was unfortunately not quantified), I have to work a lot of unpaid project overtime? And the girlfriend wants to stay at €1,400 part-time after parental leave? Daycare will surely still cost, and the household income remains < €4,000. The plot is not simple; it would have to be significantly smaller and adapted.